public with sharing class POprepopuate {
public Account acc;
public Purchase_Object__c po{get;set;}
    public POprepopuate(ApexPages.StandardSetController controller) {
       prepopulate();
    }

    public void prepopulate(){
    acc = [select Id,Name,Oracle_Account_Number__c from Account where id = :ApexPages.currentPage().getParameters().get('id')]; 
    System.debug('Check the account' + acc); 
    if(acc != NULL){     
      po.Account_Name__c = acc.Name;       
    }
   }
 }